timekeeping: Prepare for cross timestamps on arbitrary clock IDs

PTP device system crosstime stamps support only CLOCK_REALTIME, which is
meaningless for AUX clocks. The PTP core hands in the clock ID already, so
prepare the core code to honor it.

 - Add a new sys_systime field to struct system_device_crosststamp which
   aliases the sys_realtime field. Once all users are converted
   sys_realtime can be removed.

 - Prepare get_device_system_crosststamp() and the related code for it by
   switching to sys_systime and providing the initial changes to utilize
   different time keepers.

No functional change intended.

Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: David Woodhouse <dwmw@amazon.co.uk>
Tested-by: Arthur Kiyanovski <akiyano@amazon.com>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20260529195557.846634842@kernel.org

1 files changed, 6 insertions, 1 deletions

diff --git a/include/linux/timekeeping.h b/include/linux/timekeeping.h
index 2d9860ecb285..017599d04eb4 100644
--- a/include/linux/timekeeping.h
+++ b/include/linux/timekeeping.h
@@ -319,13 +319,18 @@ struct system_counterval_t {
  * @device:		Device time
  * @sys_counter:	Clocksource counter value simultaneous with device time
  * @sys_realtime:	Realtime simultaneous with device time
+ * @sys_systime:	System time for @clock_id
  * @sys_monoraw:	Monotonic raw simultaneous with device time
  */
 struct system_device_crosststamp {
 	clockid_t			clock_id;
 	ktime_t				device;
 	struct system_counterval_t	sys_counter;
-	ktime_t				sys_realtime;
+	union {
+		/* realtime goes away once all users are converted */
+		ktime_t			sys_realtime;
+		ktime_t			sys_systime;
+	};
 	ktime_t				sys_monoraw;
 };